One of the most important questions in evolutionary biology is how new traits originate. To find answers, this proposal focuses on the origin of self-??fertility in some species of roundworms. These species produce hermaphrodites that look like females, but make their own sperm and fertilize their own eggs. Surprisingly, thesetypesofhermaphroditesevolvedindependentlyonmanyseparateoccasions.Thus,geneticcomparisons betweendifferentroundwormspeciesprovideanaturalsystemforlearningaboutevolutioninthelaboratory. Two of these species are particularly useful. C. nigoni is male/female , but C. briggsae makes hermaphrodites. Despitethisdramaticdifference,thetwospeciesaresocloselyrelatedthattheycanmateandproducefertile offspring.Hence,geneticcomparisonsbetweenthemshouldrevealwhichgenescontrolself-??fertility. The central hypothesis in this proposal is that new traits like self-??fertility originate in three steps. The first produces preconditions that are required for the new trait. The unequal distribution of these preconditions means that some species are more likely to evolve a particular trait than others. The second step involves a precipitatingchangethatproducesthenewtraitinanunrefinedform,byco-??optingoldergeneticprocesses.The third stage consists of reinforcing changes that optimize the trait. The mutations controlling each step can be identifiedbytheirbehaviorinthetestsdescribedbelow. Aim#1:Determinehowsexualdevelopmentiscontrolledinmale/femalespeciesofroundworms.These resultswilldefinetheancestralsex-??determinationpathwayandidentifypreconditionsforself-??fertility. Aim #2: Identify C. briggsae genes that are necessary and sufficient for XX spermatogenesis. Swapping genes between female and hermaphroditic species will reveal which genes are sufficient to make herma-?? phrodites. These genetic changes might have precipitated the transition to self-??fertility. Other genes that only affectthenumberorqualityofsperminhermaphroditesprobablyhelpedoptimizethistraitafteritfirstarose. Aim#3:Useforwardandreversegeneticstolearnhowself-??fertilityisregulatedinC.tropicalis.Dissecting thegeneticregulationofself-??fertilityinathirdspecieswilltestthesepredictionsabouthowitiscontrolled. In roundworms, the Gli protein TRA-??1 plays a key role in the control of sexual development, and interacts withothermedicallyrelevantgenestocontrolwhichgermcellsformeggsandwhichmakesperm.Thus,these studieswillprovidenewinformationabouttheregulationofgenesthatplayimportantrolesinhumanhealth.